In any mobile communication system, such as a GSM network active calls conducted between a mobile station and a base station need to be handed over to a different base station as the mobile station moves between different coverage areas, or cells. Depending on how each cell is defined, handover may require the active call to be re-routed simply through a different base station transceiver BTS, through a different base station controller BSC or through a different mobile services switching center MSC. Handover may also be necessary when capacity problems are met in any one cell.
Handover necessitates a certain amount of operation and maintenance, such as defining neighbouring cells, as well as the base station controller BSC and mobile services switching center MSC that controls the cell, defining which cell frequencies should be measured and what threshold value to use to initiate handover. In a conventional GSM network the base station controller BSC sends a mobile station a list of frequencies to be measured. Two lists may be sent out, a first list being used for idle mode, such as when the mobile station is roaming, and a second used for active mode when a call is ongoing. This second list defines which frequencies the mobile station should measure and report back on. These lists contain a set of values that refer to absolute radio frequency channel numbers ARFCN of neighbouring cells. In addition to these frequency channel numbers the base station controller BSC also knows base station identity codes BSIC of all neighbouring cells. The mobile station measures the frequencies defined by these channel numbers and reports these measurements to the base station controller. In practice, the mobile station will report on only the six best measurement values and only for those cell frequencies on which that the mobile station can synchronise and consequently receive an identity code relating to the base station (BSIC). The measurement report sent back to the base station controller BSC by the mobile station MS includes a reference to the absolute radio frequency channel numbers ARFCN, the base station identity codes (BSIC) and an indication of the uplink signal strength. In fact the report does not specify the exact absolute radio frequency channel numbers ARFCN but rather refers to the position this number occupied in the measurement list. On the basis of this report, the base station controller BSC decides whether handover is necessary and to which cell. The initiation of handover is performed according to the standard GSM mechanism for each vendor. Specifically, a message is sent by the base station controller to the mobile services switching center MSC connected to the base station controller BSC indicating that handover is required. This message contains a cell identifier, the cell global identifier CGI, which defines the mobile country code, mobile network code and location area code for the cell to which handover is requested. The cell global identifier CGI is fetched by the base station controller from a list using the base station identification code BSIC and absolute radio frequency channel number ARFCN obtained for the cell. With this cell global identification CGI the mobile services switching center MSC is able to determine which other MSC handles the cell defined by the CGI value.
Recently proposals have been made to extend conventional cellular networks by including access networks that utilise a low power unlicensed-radio interface to communicate with mobile stations. The access networks are designed to be used together with the core elements of a standard public mobile network. The access network is constructed so that the core elements, such as the mobile switching centers MSC, of the public mobile network views the unlicensed-radio access network as a conventional base station controller BSC. Such an access network and the mobile station for use with this access network is described in European patent application No. EP-A-1 207 708. The content of this application is incorporated herein by reference. The low power and, resultant low range of the unlicensed-radio interface means that several such access networks may be provided in relatively close proximity, for example one access network per floor of an office building. The access network preferably also includes a fixed broadband network which connects to a mobile services switching centre (MSC) of a conventional GSM public mobile network. This greatly facilitates the installation of the access network, permitting a subscriber to install the access network in his own home himself, for example. Suitable unlicensed-radio formats include digital enhanced cordless telecommunications (DECT), wireless LAN and Bluetooth. An adapted mobile handset capable of operating over both the standard air interface (e.g. the Um interface) and the unlicensed-radio interface means that the subscriber requires only one phone for all environments.
The problem when including one or more unlicensed-radio access networks in a conventional public licensed mobile network such as a GSM, UMTS or CDMA2000 network is that handover from the public licensed mobile network to the unlicensed-radio access network greatly increases the necessary operational and maintenance measures required in some cases to unacceptably high levels. Depending on the number of unlicensed-radio access networks present, the number of access points could amount to thousands or tens of thousands. Defining these access points in the relevant elements of the public licensed mobile network would be a time-consuming and costly task. In addition, several unlicensed-radio access points may be located in the same public licensed mobile network cell. It may thus not be possible for the base station controller to compile a list of all possible frequencies that require measurement within the cell in addition to those of the cells adjacent the public licensed mobile network. In addition the ease of installation of the individual access points of an unlicensed-radio access network means that the number and location of these access points could be constantly changing. Each change would require the public licensed mobile network to be updated on the new location of the access points.